NiCad question

[Roff]

I have hair clippers with a series stack of three 1.2V, 600maH AA cells. It seemed like it wasn't holding a charge, so like any good engineer, I opened it up and made some measurements.
The batteries were discharged when I opened it up, so I put the charger on it. It started charging at about 90ma, and after about 3 hours the rate was about 65ma. I then measured the open circuit voltage at 4.2V.
I disconnected the charger and turned on the clippers. The motor drew about 700ma, quickly dropping to about 600ma as the voltage plateau'ed at 3.6V, as is to be expected.
This is where it got strange. After only about 9 minutes of running, the voltage abruptly (within about 15 seconds) dropped to about 2.1V, and the current to about 540ma (realize that the motor is not a constant resistance load). It ran in that condition for a few minutes, until I turned it off. It was too slow to cut hair, and I suspect that under load it would have stopped as soon as the voltage started to drop.
The open circuit voltage quickly recovered to 3.6V, but dropped to 2.1V again almost immediately when I turned it back on.
It almost seems as if the voltage of one of the cells goes to zero abruptly, or maybe internal resistance goes up suddenly. Whatever the cause, I'm thinking that a new battery is in order. Any thoughts?
BTW, the charger is crude. It has a series 10ohm resistor, plus the output resistance of the wall wart. Are NiCads tolerant of overcharge? I found an Eveready document that implied that theirs are, so long as you keep the charge rate below 0.1 times the hour rating (which mine almost does), but I'm interested in personal the experiences of you guys.

 

[k7elp60]

Hi Ron,
The nominal cell voltage of Ni-cad batteries is 1.2V, but when charging they go to very close to 1.5V.

Looks like you have one bad cell. They can and do short.
I would recommend that you change all the cells as if you just replace the bad one, there is no definate time when another or the remaing cells too
will go bad.
Ni-cad batteries are like other rechargeable batteries. Each time they are cycled(charged and discharged) they loose a small amount of their capacity. So the overall remaining capacity in the two other cells is questionable.
I have built a large quantiy of Ni-cad chargers for many different capacity of batteries. My experience has been to charge them at C X .095, this way they can remain on the charger indefinately. The charger in this case must be capable of supplying 1.5V per cell. This is at nominal room temperature.

An interesting thing happens when Ni-cad batterys are near full charge.
1. The cell voltage peaks at near 1.5 volts then drops off slightly
2. The cell temperature increases
3. The internal pressure of the cell increases.

Ned

 

[audioguru]

Hi Ron,
One of the cells has much less capacity than the others. Each time it is charged, it reaches its fully-charged status before the others. Then it continues in overcharge while the others are still charging normally.

Overcharging a Ni-Cad produces heat and pressure. Perhaps the weak cell vented due to the pressure then got worse during each overcharge.
Of course when it vents then some of its capacity is released.

Also, most likely the weak cell fully discharged while the clipper was running then had charge-reversal since the other good cells discharged their current through it in reverse. Cell charge-reversal also causes heat and pressure, and maybe venting.

Energizer has confliciting statements in their applications manual. For trickle charging, they say that the 1 hour discharge rate divided by 30 to 50 is optimum for best performance, the 1hr rate divided by 10 is "safe" and the 1hr rate divided by 20 shows a chart of it losing about 30% of its capacity after 2 years of overcharge.

Replace the cells with Ni-MH ones that have a much higher capacity. Then reduce the charging current as low as you can. You don't use that clipper every day so let it slowly charge and have a lower overcharge current. For their Ni-MH cells, Energizer recommends a trickle-charge current that is its 1hr discharge rate divided by 100. The cells would be fully charged in about 1 week. :lol:

 

[mstechca]

...The open circuit voltage quickly recovered to 3.6V, but dropped to 2.1V again almost immediately when I turned it back on.
It almost seems as if the voltage of one of the cells goes to zero abruptly, or maybe internal resistance goes up suddenly. Whatever the cause, I'm thinking that a new battery is in order. Any thoughts?

I went through about 5 or 10 non-rechargeable 9V batteries.

I notice that the 9V batteries that measure under 6V tend to die the same way. 6V, 5V, 4V, etc. I even notice it the most in my regen.

Try new batteries.

 

[audioguru]

I notice that the 9V batteries that measure under 6V tend to die the same way. 6V, 5V, 4V, etc. I even notice it the most in my regen.

A 9V battery that measures 6V is finished. That's why many circuits powered by a 9V battery use a low-dropout 5V regulator and a "low-batt" indicator.

 

[audioguru]

I went through about 5 or 10 non-rechargeable 9V batteries.

You wasted a lot of money by not using a rechargeable 9V battery.

 

[Roff]

Ned and $crooge, thanks for the informative replies.

 

[Tim B]

Since there is only 3 cells, it would hardly be a problem to just replace all of them with three new cells all of the same cpacity.
With new cells, it is often a good idea to charge them fully (15 hours at Capaity *.1 or 40-60ma) then discharge them to 1.1 volts per cell (usually at about 120ma) obviously the time of this discharge will vary dependant on the capacity.

For instance, a 240mah cell discharged at 120ma will last about 2 hours.

If possible, it is often a good idea to idea to connect the batteries to a computer via an ADC. That way you can monitor the voltage history of the pack.

Tim B.

 

[Roff]

Since there is only 3 cells, it would hardly be a problem to just replace all of them with three new cells all of the same cpacity.
With new cells, it is often a good idea to charge them fully (15 hours at Capaity *.1 or 40-60ma) then discharge them to 1.1 volts per cell (usually at about 120ma) obviously the time of this discharge will vary dependant on the capacity.

For instance, a 240mah cell discharged at 120ma will last about 2 hours.

If possible, it is often a good idea to idea to connect the batteries to a computer via an ADC. That way you can monitor the voltage history of the pack.

Tim B.

Thanks, Tim. What is the purpose of that procedure?

 

[Tim B]

Basically, cycling cells allows them to recover to somewhere near their optimimum capacity. Effectively allowing the chemicals to move to the right places and reduce the amount of crystalline structure.

The trick is not to over-charge (or over-discharge) the cells, but do everything over a prolonged period of time. (ie. 15 hour charge, then discharge at low current). For Ni-Cads running large electric motors (ie. High Charge/Discharge currents over short times) Cycling will seriously prolong the life of the cells. One proper cycle in every 10 is probably about right for high-power applications.

In low-power applications, a discharge to 1.1v/cell then a 15 hour charge is usually good enough, then do a cycle every 50 or so, or as soon as you're worried about the cells.

Tim B.